According to Wikipedia, capacitive sensing is a technology for detecting proximity, position, etc., based on capacitive coupling effects. Capacitive sensing is a human interface device (HID) technology, used for example to replace the computer mouse. Capacitive sensors can be found in many popular consumer products such as laptop trackpads, MP3 players, computer monitors and cell phones, but they are certainly not limited to these applications. Capacitive sensors can be constructed from many different materials, such as copper, Indium tin oxide (ITO) and printed ink. Copper capacitive sensors can be implemented on standard FR4 PCBs as well as on flexible material. ITO allows the capacitive sensor to be up to 90% transparent (for single layer solutions). The size and spacing of the capacitive sensor array are both very important to the sensor's performance. In addition to the size of the sensors, and their spacing relative to the ground plane, the type of ground plane used is very important. Since the parasitic capacitance of the sensors is related to the electric field path to ground, it is important to choose a ground plane that limits the concentration of electric field lines without a conductive object present.
A capacitive surface is defined as a surface, sensitive to touch of one or more objects, where the touch objects' locations can be computed using a measuring device connected to it.
There are two common ways for implementing a capacitive surface. The first is trans-capacitance or projective capacitive, where the object (finger, conductive stylus) alters the trans-capacitance coupling between row and column electrodes, which are scanned sequentially. Projective capacitance technology-based touch panels are capable of detecting multi-touch.
The second way for implementing a capacitive surface is termed Absolute Capacitance or Surface Capacitance where the object (finger, etc.) is sensed by the presence on the first and perpendicular axes of the surface.
Sometimes, a relative positioning is required. Subtraction of a preceding absolute position from the present absolute position yields the relative motion of the object/finger during that time.
Published U.S. Pat. Nos. 6,188,391, 7,129,935, 7,292,859 and 7,218,124 describe different patterns for two-way surface capacitive pads and methods to detect coordinates for single touch point only.
Published U.S. Pat. No. 5,825,352 describes a multiple finger contact sensing method for emulating mouse buttons and mouse operations on a touch sensor pad, and specifies that touch sensing technology is capacitive sensing, in which the location of a finger (or in some instances another object such as a stylus) over a sensing device is determined by virtue of variations in capacitance under and around the location of the finger. Typical applications of capacitive surfaces are touch screens and touch pads (or track pads) which employ a matrix of row and column electrodes and detect, for example, either the trans-capacitance between row and column electrodes or the effective capacitance to virtual ground. Some touch sensitive devices are known to use interpolation for a more precise identification of the location of a finger or stylus.
A typical limitation of the above prior art is the ability to sense only one finger at a time. Cursor movement is straightforward with one finger, and tapping of a finger on the surface of the pad can be detected and acted upon in a manner similar to detecting the actuation of a button on a mouse. Single and double taps can be used as simple equivalents of single and double mouse clicks.
With a single-finger touchpad, the click and drag function is more difficult. With single finger detection, dragging has been implemented with schemes such as uptap (finger lifted and placed down again quickly), tap-and-a-half, and sticky drag (drag lock turns on automatically after the finger is placed in one location without moving for more than a certain time, such as one second). All of these methods take more time and/or more finger motions than it takes to perform the equivalent function with a mouse, and are not intuitive to users familiar with electronic mice. Prior art touch pads are thus less attractive for general use than a mouse.
The above U.S. Pat. No. 5,825,352 claims “a method for detecting the operative coupling of multiple fingers to a touch sensor involving the steps of scanning the touch sensor to (a) identify a first maxima in a signal corresponding to a first finger, (b) identify a minima following the first maxima, (c) identify a second maxima in a signal corresponding to a second finger following said minima, and providing an indication of the simultaneous presence of two fingers in response to identification of said first and second maxima”.
However, the presented method does not solve the ambiguity problem of multi-touch points (for example when the points are located in diagonal). The method presented in U.S. Pat. No. 5,825,352 is able to solve X and Y coordinates for multiple touch points when all points are located on the same axis, either X or Y. Another example of ambiguity is four touch points located in corners of square—the described method of U.S. Pat. No. 5,825,352 is not capable of distinguishing between four, three or two touch points. FIG. 1 shows several touch scenarios, all detected by the same row and column sensors.
The disclosures of all publications and patent documents mentioned in the specification, and of the publications and patent documents cited therein directly or indirectly, are hereby incorporated by reference.